This invention relates generally to communication systems, and, more particularly, to a permutation zone for coordinated multi-point systems.
Base stations in wireless communication systems provide wireless connectivity to users within a geographic area, or cell, associated with the base station. In some cases, the cell may be divided into sectors that subtend a selected opening angle (e.g., three 120° sectors or six 60° sectors) and are served by different antennas. The wireless communication links between the base station and each of the users typically includes one or more downlink (DL) (or forward link) channels for transmitting information from the base station to the mobile unit and one or more uplink (UL) (or reverse link) channels for transmitting information from the mobile unit to the base station. The uplink and/or downlink channels include traffic channels, signaling channels, broadcast channels, paging channels, pilot channels, and the like. The channels can be defined according to various protocols including time division multiple access (TDMA), frequency division multiple access (FDMA), code division multiple access (CDMA), orthogonal frequency division multiple access (OFDMA), as well as combinations of these techniques. The geographical extent of each cell may be time variable and may be determined by the transmission powers used by the base stations, access point, and/or mobile units, as well as by environmental conditions, physical obstructions, and the like.
Mobile units are assigned to base stations based upon properties of the channels supported by the corresponding air interface. For example, in a traditional cellular system, each mobile unit is assigned to a cell on the basis of criteria such as the uplink and/or downlink signal strength. The mobile unit then communicates with that serving cell over the appropriate uplink and/or downlink channels. Signals transmitted between the mobile unit and the serving cell may interfere with communications associated with other mobile units and/or cells. For example, mobile units and/or base stations create intercell interference for all other sites that use the same time/frequency resources. The increasing demand for wireless communication resources has pushed service providers towards implementing universal resource reuse, which increases the likelihood of intercell interference. In fact, the performance of modern systems is primarily limited by intercell interference, which dominates the underlying thermal noise.
Intercell interference can be reduced in several ways, for example through frequency planning, soft handoff, or beamforming multiple antennas. For example, most mobile systems employ sector specific frequency and/or temporal permutation, hopping, or scrambling in the down link (DL). This is generally beneficial to aid in inter-sector interference averaging especially in frequency reuse one systems, e.g., OFDMA and CDMA systems.
A new type of system architecture, referred to as Coordinated Multi-Point (CoMP), has been proposed where multiple base stations may concurrently communicate with a single mobile station to improve performance. Exemplary preliminary standards for such approaches are LTE-Adv (Rel. 10) and IEEE 802.16m. A limitation of current techniques for reducing inter-cell interference is that such techniques are specifically designed to prevent a mobile station from recognizing signals from other cells.